Check group id existence in storage implemented

Issue #29
ok. but '(un)safety' is still far from sane in here... 

As far as I see now, the group mapping to C3 is created upon group approval: `ApproveGroup#createAndApprove` method. 
And upon approval we call `GroupService#createGroup` method that actually creates mapping (but doesn't check again that it's even possible)... d'Oh, keeping two storages (localDB and C3) in sync state is hard in general. But currently our logic to do that is scattering across different controllers and etc.

That's why i'd suggest to consider:
1) Have preferably one place/component where all validation/creation of C3 mappings is done.
2) use uuid for mappings